Scent of a stickleback

We all know why we find certain people attractive and want to form relationships with them. Those special people might be more than usually compassionate, intelligent or funny — or might be physically well-endowed in some way. And neurologists know which areas of the brain become active when we meet that special someone — the ventral striatum, a brain area linked to the anticipation of reward.

But at a deeper level, why do we prefer some people as sexual partners to others? The answer, says a team of German scientists, can be found in fish. Sticklebacks, to be precise.

According to the theory of sexual selection, there are several reasons why animals choose between different prospective mates, as opposed to mating with random members of the opposite sex. One hypothesis says that “mate choice” functions to avoid inbreeding. If sexual partners were selected at random, there would be a high probability of mating with a close relative (a bad idea genetically, not to mention psychologically). By being choosy in mating preference, you might also be able to produce attractive offspring, or if you select a rich partner, gain access to his (or her) resources.

Another hypothesis says that mate choice occurs to maximize the chances of offspring having genes that protect them from disease. The stickleback research, published today in Nature, supports this “good genes” hypothesis.

The genes that are particularly important in the vertebrate immune system belong to a family known as the major histocompatibility complex. The MHC makes molecules that stick to invaders in the body and carry them to T cells, which make antibodies to fight them. The reason people get sick is because their immune system is slow to recognize invaders, and therefore slow to make antibodies against them. (HIV is a hard virus to fight because it keeps changing its surface proteins, so that antibodies, built to recognize a certain virus-type, are soon outdated.)

So having an MHC that can recognize many different invaders is clearly going to be a good thing — it’s like having detailed intelligence of the tactics and weaponry of your enemies before a war, compared to entering battle knowing nothing about who you are fighting.

Fine so far, but how do you find out a potential partner’s MHC status? You can’t, as you might do with other mate-choice criteria, just look in his wallet or in his jeans. But you can smell him. MHC genes affect body odor. In mice — and even in humans — there is evidence that females can (unconsciously) perceive MHC differences through scent alone.

If mate choice occurs to boost the immune system in this way, then sexual partners should have a high diversity of MHC genes. This is what Thorsten Reusch and colleagues, at the Max Planck Institute for Limnology, in Plon, Germany, were looking for in sticklebacks. In flow chambers, egg-laden stickleback females were allowed to choose between tanks containing water that had been “scented” by males with high or low numbers of MHC genes. The females preferred the scent of males that had the most diverse MHC. The researchers were quite sure that the females were choosing tanks with the “intention” of selecting a mating partner, because most of them spontaneously spawned after the experiments, even in the absence of males.

What does that mean for us? Maybe quite a lot.

In 1995, Claus Wedekind, a Swiss zoologist then working at Bern University, performed the now-famous “sweaty T-shirt” experiment. Women were asked to smell cardboard boxes, each containing a sweaty T-shirt from a different man, and choose the box they liked best. All subjects were “typed” for their MHC status. It turned out that women preferred the scent of T-shirts worn by men who had MHC genes most different from their own.

The stickleback experiments went further, because the researchers excluded the possibility that females were simply trying to find males with MHCs that were different, as in the sweaty T-shirt study: Given the choice between males with different MHCs and those with more diverse MHCs, females chose diversity.

But there’s a twist in the tale. Female sticklebacks with an already high MHC count were less attracted to high-MHC males, Reusch and colleagues found. Diversity is good, but if your MHC diversity is really high, you are at greater risk of autoimmune diseases and your T cells won’t work as well. Female sticklebacks seem to be able to “count” the genetic diversity of a potential mate’s MHC and decide to mate or not accordingly.

The counting strategy “is ubiquitous in vertebrates with several MHC class-IIB loci,” write the researchers. That means us. So while you may think you are drawn to someone by their intelligence, sense of humor or compassion, chemical attraction is probably also at work. Next time you meet a potential mate, make sure you don’t have a blocked-up nose.